Embodiments of the invention will be described while particularly useful in active matrix OLED displays although applications and embodiments of the invention are not limited to such displays and may be employed with other types of active matrix display and also, in embodiments, in active matrix sensor arrays.
Organic Light Emitting Diode Displays
Organic light emitting diodes, which here include organometallic LEDs, may be fabricated using materials including polymers, small molecules and dendrimers, in a range of colours which depend upon the materials employed. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of so called small molecule based devices are described in U.S. Pat. No. 4,539,507. A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-colour pixellated display. A multicoloured display may be constructed using groups of red, green, and blue emitting sub-pixels. So-called active matrix displays have a memory element, typically a storage capacitor, and a transistor, associated with each pixel (whereas passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image). Examples of polymer and small-molecule active matrix display drivers can be found in WO 99/42983 and EP 0,717,446A respectively.
It is common to provide a current-programmed drive to an OLED because the brightness of an OLED is determined by the current flowing through the device, this determining the number of photons it generates, whereas in a simple voltage-programmed configuration it can be difficult to predict how bright a pixel will appear when driven.
Background prior art relating to voltage programmed active matrix pixel driver circuits can be found in Dawson et al, (1998), “The impact of the transient response of organic light emitting diodes on the design of active matrix OLED displays”, IEEE International Electron Device Meeting, San Francisco, Calif., 875-878. Background prior art relating to current programmed active matrix pixel driver circuits can be found in “Solution for Large-Area Full-Color OLED Television—Light Emitting Polymer and a-Si TFT Technologies”, T. Shirasaki, T. Ozaki, T. Toyama, M. Takei, M. Kumagai, K. Sato, S. Shimoda, T. Tano, K. Yamamoto, K. Morimoto, J. Ogura and R. Hattori of Casio Computer Co Ltd and Kyushu University, Invited paper AMD3/OLED5-1, 11th International Display Workshops, 8-10 Dec. 2004, IDW '04 Conference Proceedings pp 275-278. Further background prior art can be found in U.S. Pat. No. 5,982,462 and in JP2003/271095.
FIGS. 1a and 1b, which are taken from the IDW '04 paper, show an example current programmed active matrix pixel circuit and a corresponding timing diagram. In operation, in a first stage the data line is briefly grounded to discharge Cs and the junction capacitance of the OLED (Vselect, Vreset high; Vsource low). Then a data sink Idata is applied so that a corresponding current flows through T3 and Cs stores the gate voltage required for this current (Vsource is low so that no current flows through the OLED, and T1 is on so T3 is diode connected). Finally the select line is de-asserted and Vsource is taken high so that the programmed current (as determined by the gate voltage stored on Cs) flows through the OLED (IOLED).
There is, however, a need for improved pixel driver circuits.